Biden’s advancing dementia is certainly enough to slow him down in the general election. He will be a lackadaisical campaigner, needing days of rest between small brief rallies that will tax his endurance. Trump will criss-cross the country holding hours- long, large, high-energy, rallies. Democrats may accept the logic of voting for Biden, but he does suffer an enthusiasm gap against Bernie. The gap in enthusiasm will be even larger against Trump. Lots of marginal Democrats will fail to show up at the polls to pull a lever for Biden.
Biden will fare poorly debating Trump. Biden could misstate key facts or get lost in his own train of thought.
Biden is also immensely vulnerable to questions about his record in the Ukraine and what he will do if his son gets a cushy job for a foreign country. His Democrat opponents never challenged him on the family corruption issue and he has never had to answer tough questions about it on a debate stage. He grew testy and combative when a voter asked him about it.
Biden at the top of the ticket will help Democrats in down ballot races. But he stands less of a chance than Hillary of winning against Trump.